What happened before:

 

For almost a year I have a vintage turntable from Perpetuum Ebner at home.

It's not mine. It belongs to someone that asked me to fix it. And it turned out that fixing the motor would cost too much.

I asked the owner to collect the tt. That hasn't happened yet and the machine is collecting dust at my home.

 

So I'll take the freedom to attempt a non-intrusive repair with modern components. I'm also thinking about making it an Enchanted Objects Design Challenge.

 

In post 1 I present the turntable, talk about my previous repair attempts and brainstorm on some modding ideas.
In post 2 I'm measuring up the different gears, pulleys and wheels, and I calculate the speed of the original motor.

 

This is the first post on motor control. I'm reviewing the operation of the Infineon DC Motor Control Shield.

I'm going to use this shield to drive a replacement turntable motor.

 

Photo 24-02-15 11 52 13.jpg

 

The shield comes with two half-H-Bridge drivers of type BTN8982. The drivers can be controlled independently.

That gives the option to use it for two unidirectional motors or for one bidirectional one.

Even though I don't need two directions, I think I'm going to wire the bridge for bidirectional use. I may build in a satanic message detector into the turntable.

 

Driving the motor

 

The driver is controlled by two pins. The IN and INH pins are the ones we need to drive to get the desired behavior.

 

 

 

 

hbridge_pkg.jpg hbridge_pins.jpg

 

Subset of the truth table for driving the bridge. This is the explanation on what happens when you drive the INH and IN pins.

You activate the bridge with INH, You select the speed by providing a PWM signal with correct duty cycle to IN.

truth.jpg

 

 

Fine-tuning the slew rate

 

The SR pin is used to tweak the slew rate. I may use this if the controller interferes with the turntable's audio. I want to go for a very slow rise and fall time.

I don't have the BOM for the shield, but measured the resistor between SR and ground in circuit. I get 510 Ohm when I measure in-circuit.

 

hbridge_slewrate.jpg

 

The BTN8982 data sheet defines the slew rater behavior:

hbridge_slewrate_2_c.jpg

 

Unfortunately for lazy me, the characteristics for the on-board Rsr are not documented in the datasheet. So I'll either have to pull out my calculator or my scope.

 

...

 

OK. Decision made. I'll use the scope .

All measurements below a re made by driving the input with my function generator's TTL output.

rise time.bmp

 

I've measured in half bridge mode on channel 1. The rise time of my signal, with a 15V supply and 150K load (I also used a 22R load for a brief time and it didn't change the rise time) is 380 ns.

The datasheet says 'We recommend a PWM-period at least 10 times the rise-time'.

That means:

Rise-time = fall-time = 380ns.

=> T-PWM = 10 * 380ns = 3.8μs.

=> f-PWM = 263kHz.

However, when I tested this, the regulator started to perform badly at 60KHz and flatlined at 65kHz (see the two captures below):

60kHz.bmp 65kHz.bmp

 

The below capture gives a correlation between input and output, measured at 56kHz, duty cycle 58.7%:

td f(LS)

td f(LS).bmp

tf(LS)

tf(LS).bmp

td r(LS)

td r(LS).bmp

tr(LS)

tr(LS).bmp

 

Note: the shield schematic

The shield documentation doesn't specify the actual components and schema.

I've been probing the components with my meter and found that the shield is the literal implementation of the full H-bridge reference design form the datasheet:

referencedesign.jpg


In the next post I'm going to use the enhanced PWM module of a The specified item was not found. Hercules LaunchPad to test drive the motor.


 

Related posts
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 1
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 2 - Calculating the Motor Speed
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 4 - Hercules LaunchPad Enhanced PWM try-out
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 5 - Yes I Can
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 6 - Speed Adjustment with Variable Duty Cycle
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 7 - Make Speed Sensor from Scrap Parts
Vintage Turntable repair: Can I fix a Perpetuum Ebner from 1958 - part 8 - Sample the Motor Speed with Microcontroller